Coatings and surface modifications are used for a variety of applications including environmental protection, metal refinement, lubrication between moving parts, and maintenance. For example, large metal surfaces, antennas, and windows are coated to prevent the build-up of snow, ice, and fog. Boats are often treated with an anti-fouling paint to protect against materials that accumulate on wetted structures. Building and glass surfaces can be modified to become anti-soiling and stain resistant, respectively. Surface modifications can also render automobile windshields, airplane canopies, and optical devices self-cleaning. The advantages of appropriate surface coatings and modifications are well understood and appreciated. Recently, a number of recognized techniques for surface treatment use nanomaterials to produce effects that are more efficient and longer lasting than conventional coatings. For example, metallic stainless steel coatings sprayed with nanocrystalline powders demonstrate increased hardness when compared to traditional treatments. Hard ceramic nanocoatings made with titanium dioxide and a plasma torch renders metals very resistant to corrosion.
The extremely high ratio of surface area to volume of nanoparticles is a unique characteristic that provides for the synthesis and control of materials in nanometer dimensions. Accordingly, extensive work in the field of nanotechnology has been done to exploit new material properties and device characteristics through nanostructuring.
Among these new material properties, water-repelling hydrophobic surfaces and their production are extremely beneficial, for example, in the area of corrosion inhibition for metal, chemical and biological agent protection for non-metals, and so on. Over the past decade, research has been conducted to engineer the surface chemistry and roughness of solids to mimic the natural super-hydrophobic characteristics found in the lotus leaf. Super-hydrophobic surfaces and coatings possessing a so called “lotus leaf effect” have unique properties with very high water repellency. For example, the surfaces of many structures, such as aircraft surfaces, glass and plastics are susceptible to the buildup of ice, water, fog and other contaminants that can interfere with ordinary use. Super-hydrophobic surfaces on such structures can prevent or mitigate the buildup of ice, water fog and other contaminants by creating a microscopically rough surface containing sharp edges and air pockets in a material that sheds water well.
A super-hydrophobic surface is defined as possessing a water surface contact angle (CA) greater than 150° and a surface tension of approximately one-fourth of water. Since the surface tension of water is approximately 70 mNM−1, the coated super-hydrophobic surface tension should be no more than several mNM−1.
The first example of a super-hydrophobic surface was demonstrated in 1998 using an anodically oxidized fractal structured aluminum plate. Subsequently, engineers have developed several different textured surfaces with local surface geometries having super-hydrophobic surface CAs greater than 160°, even with octane. An example is disclosed in U.S. patent application Ser. No. 12/599,465, U.S. Publication No. 2010/0316842 A1, filed Apr. 14, 2008, for a “Tunable Surface” to Tuteja, et al., which is hereby incorporated by reference in its entirety. This application contemplates modifying surfaces to include a protruding portion to protrude toward a liquid and a re-entrant portion opposite the protruding portion to enhance the resistance/contact angle with any liquid. However, fabricating the necessary re-entrant angles and local surface geometric structures using this method is both time consuming and expensive. Specifically, the fabrication requires a Silicon dioxide (SiO2) deposition followed by a costly two-step etching process comprising reactive ion etching of SiO2 and subsequent isotropic etching of Si with the use of vapor-phase Xenon difluoride (XeF2). Furthermore, this fabrication technology is only feasible for creation of the necessary re-entrant angles in localized surface geometric structures of micron sizes (e.g., approximately 20 μm).
Additionally, while a super-hydrophobic surface can provide excellent ice repellency on a clean surface, oil, dirt, salt and other contaminants already existing on the surface could enable additional ice accumulation. Therefore, the best surface modification technology for ice repellency will impart both super-hydrophobic and super-oleophobic properties. Such surfaces would be highly self-cleaning since they would tend to shed not only oil-based contaminants, but also water-based contaminants, thereby providing additional benefits such as anti-corrosion and ease of cleaning.
Similar to super-hydrophobic surfaces, a super-oleophobic surface is defined as any surface that reduces the tendency for an oil to attach to that surface or form a film on that surface. In particular, a super-oleophobic surface possesses an oil CA greater than 150°.
In another example of super-hydrophobic surface modifications, a biomimetic procedure was used to prepare super-hydrophobic cotton textiles. This procedure is discussed further in a paper by Hoefnagel et al., for “Biomimetic Superhydrophobic on Highly Oleophobic Cotton Textiles” (Hoefnagels, H. F., Wu, D., With, G. de, Ming, W. (2007) Langmuir, 23, 13158-163), which is hereby incorporated by reference in its entirety. This publication discloses a method for creating a super-hydrophobic (i.e., having a water CA greater than 155°) cotton textile by introducing silica particles in situ to cotton fibers to generate a dual-scale surface roughness, followed by hydrophobization with polydimethylsiloxane (PDMS). Although this approach can obtain moderately oleophobic surfaces (e.g., having an oil CA of approximately 140°), the resulting coating was not super-oleophobic (i.e., having an oil CA greater than 150°) because the coverage of the silica nanoparticles was not uniform in structure (e.g., low and out of control). Furthermore, the scalability of this process is limited and excludes various surface types including, for example, the surface of aircraft wings, because the thickness and roughness of the coated layer results in clustering of the nanoparticles and yields a very irregular surface morphology in micron scale.
Accordingly, an improved system and method for low-cost surface treatments having both super-hydrophobic and super-oleophobic properties to alleviate the problems discussed above is desirable.